The present invention relates to catalysts for synthesizing polymers and, more particularly, to catalysts for synthesizing phenoxy resins suitable for use in vesicular films. Phenoxy resins, and especially highly long-chain branched phenoxy resins, are known to be well adapted as matrices for enhanced film speed vesicular films. Thus, there is described in U.S. Pat. No. 4,451,550 a highly long-chain branched non-linear phenoxy resin advantageously comprising the copolymerization product of:
(i) at least one dihydric phenol, e.g., sulfonyldiphenol; PA1 (ii) an epoxy comonomer having two epoxy functional groups, e.g., resorcinol diglycidyl ether; PA1 (iii) a branching agent comprising an epoxy or phenolic compound having a functionality greater than 2, and preferably at least 3, wherein the amount of branching agent is sufficient to provide at least 10 mole percent branch sites in the polymer resin, and, most preferably; PA1 (iv) a monofunctional phenol or epoxide as an end-blocker compound. PA1 (i) a catalyst comprising a water-insoluble complex of a phenol and a quaternary ammonium or phosphonium salt of the phenol; PA1 (ii) A solvent medium capable of dissolving the phenoxy resins and otherwise not interfering with the polymerization reaction.
The polymerization is typically carried out under alkaline catalysis using catalysts such as tetra-alkyl ammonium bases which are highly ionic and dissociated in organic solvents. Specific catalysts which can be employed include tetramethyl and tetrabutyl ammonium hydroxides and glycidyl trimethyl ammonium chloride used in conjunction with an alkali metal hydroxide e.g., potassium hydroxide, from which the precipitated potassium chloride has been filtered.
The preferred solvent for carrying out the polymerization reaction has been Methyl Cellosolve since it is capable of dissolving the polymeric product even at solids levels as low as 10% by wt. and does not otherwise interfere with the polymerization chemistry. However, Methyl Cellosolve is now recognized to be highly toxic. Additionally, due to the formation of a layer of "skin" on the surface of the barrier type polymer as the solvent is being removed, it is virtually impossible to completely remove the toxic Methyl Cellosolve from the polymeric product.
In light of the above, alternative solvent systems have been sought which, like Methyl Cellosolve, are capable of dissolving the polymeric product while not interfering with the polymerization chemistry but, unlike Methyl Cellosolve, are not toxic. To this end, the inventor has found that certain nontoxic solvents including, but not limited to 1,3-dioxolane (having a boiling point of 74.degree.-75.degree. C.) as well as alcohols such as Dowanol-PM and tetrahydrofurfuryl alcohol (having a boiling point of 178.degree. C.) are suitable for the synthesis of highly long chain branched phenoxy resins for use in vesicular films. Also suitable is 1,4 dioxane, although such is more toxic than 1,3-dioxolane.
Because high molecular weight phenoxy resins are not totally soluble in the less costly Dowanol solvent at less than circa 40% solids, it is necessary to use a solvent mixture including the better, but more costly, low boiling dioxolane solvent. Since solvent mixtures including circa 30-40% by wt. dioxolane (having a boiling point of 76.degree.-77.degree. C.) reflux at lower temperatures than Methyl Cellosolve, the polymerization reaction in these alterative solvents obviously occurs at lower temperatures compared to the Methyl Cellosolve system. The resulting lower temperatures require the employment of much higher concentrations of catalyst, even on the order of an eightfold increase, in order to economically complete the polymerization reaction. It has been found, however, that the incorporation of the higher concentrations of catalyst in the reaction mixture results in the formation of optical defects in the phenoxy resin when used to make a diazo vesicular microfilm. An investigation of this phenomenon demonstrated that those defects were caused by the crystallization of alkali metal halide salts, e.g., sodium or potassium chlorides and bromides, which were adventitiously added to the reaction mixture via the polymerization catalyst made in Dowanol-PM instead of Methyl Cellosolve. More specifically, the above-described quaternary ammonium salts used to catalyze the polymerization reaction, e.g., a quaternary ammonium hydroxide, are typically prepared as follows: ##STR1## wherein M is a sodium ion or a potassium ion, X is chloride or bromide, and R is alkyl or alkenyl groups.
The metal halide salt by-product is virtually insoluble in the alcoholic solvent and is thus typically separated from the desired quaternary ammonium salt catalyst by filtration. Nonetheless, residual levels of the by-product metal halide salts remain in the catalytic solution The presence of residual levels of metal halide salts in the catalytic solution was not significant when the polymerization was carried out in a Methyl Cellosolve solvent since only relatively low concentrations of catalyst were required to economically run the reaction at the high temperatures possible in Methyl Cellosolve. However, because of the highly increased concentrations of catalyst required when the lower boiling alternative solvents described above are employed, the appearance of optical defects in the diazo vesicular films formed from the phenoxy resin, attributable to crystallization of alkali metal halide salts, was observed.
To alleviate such problems, water-washing of the phenoxy resin to free it of the alkali metal halide salt contaminant was considered. However, the cost of such an operation would be prohibitively expensive. The removal of the salt contaminant from the catalytic solution via water washing was also considered but determined to be infeasible because the quaternary ammonium hydroxide catalysts are themselves water soluble and thus, would be removed along with the contaminant. Even conventional ion exchange techniques for removing metal halide salts from a solution had to be dismissed due to the low milliequivalent per gram capacity of such resins and thus, the prohibitively high amounts of exchange resin which would be required to prepare the large amounts of catalyst needed to sustain continuous polymer production.